In internal combustion engines the pistons employed have generally been of two types: single-piece pistons and two-piece pistons. More recently where weight has been a factor, light metals such as aluminum have been employed in both of these types of pistons. Where aluminum or other light metal is used it has often been necessary for reinforcing purposes to utilize a nickel-containing alloy insert for the top ring groove. When the aluminum piston is of the single-piece type, there has been employed a single permanent mold casting to provide pistons which have rather heavy sections from around the piston ring boss to the top of the head adjacent the combustion chamber. The heavy sections are required in order to provide adequate strength to withstand the high peak firing pressures in the cylinder. Aluminum, in addition to being light as mentioned above, has excellent heat conductivity attributes. But, in spite of these advantages, on high output engines it has been found necessary to provide oil jet cooling to the underside of the piston in order to minimize temperatures around the combustion chamber and around ring lands. The construction is such that the head section above the piston pin carries all the rings to provide the combustion seal and, at the same time, provide the oil control for suitable consumption.
In aluminum-containing piston heads the top compression ring groove insert is necessarily moved quite a bit down from the upper edge of the piston in order to provide adequate strength in the head from the top ring groove to the upper end of the piston. If the top ring groove were moved upwardly there would be a possibility of failure because of the lack of strength created by the location of this groove in conjunction with the properties of the lightweight aluminum metal between the insert and the combustion chamber cavity, this is between the insert and the upper edge of the piston.
Since aluminum alloys at normal operating temperatures in an internal combustion engine will lose strength and at high temperatures actually may become plastic, the material section between the top ring groove insert and the combustion chamber or upper part of the piston must remain adequate to prevent failure. Because the strength of the aluminum at operating temperature is low, adequate sections must also be maintained behind the ring grooves which makes it more difficult to provide adequate cooling from the oil discharged against the underside of the crown of the piston by the use of jets.
The large metal cross-sections generally require a complicated shape for the casting employed to form the pistons, and, therefore, it is necessary to carefully develop the outer contour of the piston (that part of the piston that contacts the cylinder sleeve or cylinder liner). This shape is a compound combination of ellipses, tapers and radii that call for very special tooling and grinding operations so as to maintain suitable skirt contact on the major and minor thrust surfaces in the worst possible operating conditions. These peculiarly finished contours, as previously stated, are required to compensate for the basic cross-sectional differences in the casting and the changes in dimension resulting from temperature increases.
Piston configuration is complicated by another heat related characteristic of aluminum. Aluminum has a rather high expansion rate, and, because the upper portion or head of the piston is the hottest and is supported from the lower skirt which is the coolest portion, a rather wide clearance between the piston ring lands and the bore will exist when cold, but the clearance will diminish under the heat of operation. This means that the piston rings will have to be of a design and shape to cope with the temperature and clearance variations (radial and axial) and still maintain the compression seal and lubricant seal desired for an acceptable ultimate operation life.
As noted above when the top part or head of the piston is made of aluminum it has been necessary to provide a nickel-containing alloy insert between the aluminum of the head and the topmost ring. Because the top ring is exposed to the highest temperature and pressure, it cannot survive satisfactorily when working directly against aluminum. Also, the aluminum groove wears badly when the piston ring acts directly on the aluminum. By employing the nickel-containing alloy insert the necessary load-carrying capacity and wear resistance can be achieved. As mentioned before, due to the size and shape of the aluminum crown this top ring is located further from the top of the piston than in a piston made with heavier metal which in the case of the aluminum means a relatively large unoccupied volume exists above the top ring land and between the piston and the cylinder liner. This is not desirable, because it adds heat to the top of the piston above and around its side down to the top ring land. Furthermore, because of the clearance between the crown above the ring top land and the bore, the piston usually collects excessive amounts of carbon which tends to close the clearance and eventually creates disturbances between the piston and the cylinder liner bore. In addition, there is an adverse effect on emissions since the exhaust gases will contain increased amounts of hydrocarbons.
To overcome some of the problems discussed above, a two-piece piston assembly is used where the upper piece or head is made of cast iron, malleable iron or similar material with the lower or skirt portion, sometimes called the cross head, being made of aluminum-containing metals. Because of the increased strength of iron and iron alloys at high temperatures, the upper section can be made appreciably thinner throughout and does not require the piston ring inserts since the iron itself has suitable strength at the high temperatures. In addition, the top ring can be moved further upwardly when compared to its position on the aluminum piston head and as far as practical to the point where only the strength between the top ring groove and the upper portion of the head need be considered. Since the ring carrier portion of the piston is iron which has a relatively low expansion rate it can, of course, be made to larger diameters in a given cylinder liner thus minimizing the clearance changes between the piston head and the cylinder liner and the resultant changes in the axial and radial clearances. The construction of the cast iron ring carrier section is such that it is retained on the assembly by the piston pin in such a fashion that it is free to move about the pin axis irrespective of the position or rocking of the skirt. This means that the ring carrier should be much more stable, since it will be guided by the bore itself and not subjected to the thrust loads from the gas pressure relating to the cross head section of the assembly. Since the iron upper portion can be so much thinner, oil cooling will be enhanced, since it can contact a greater exposed surface area, provide cooling through the thinner sections and reduce the temperature gradient across the piston walls.
The lower section or cross head part of the piston can be made of aluminum-containing metal and is essentially a cylinder except for some bosses required for support around the piston pin. Since there is no gas loading on the skirt and inertial loading is at a minimum, the contact area between the aluminum skirt section and the pins can be minimized. Again because the skirt is separate from the head, it will not be subjected to heat transfer by conduction from the head section. Therefore, even though the skirt is aluminum it can be fitted more tightly to maintain a closer clearance with the cylinder liner under all operating temperatures. The need for complicated shapes on the cross head or aluminum thrust section will therefore be minimized.
However, even under these circumstances it may often be necessary to increase the cooling for the smaller and thinner top section to avoid head cracking and top ring scuffing which might otherwise result from the insufficient cooling provided by the normal continuous spray of oil to the underside of the piston crown or head.
Additional cooling of the crown of two-piece pistons, as well as one-piece pistons, has been provided by what is sometimes referred to as the cocktail shaker design. In these pistons an oil-retaining section or cup having an open upper surface is provided in the head section of the piston. Oil is retained in the cup and splashes or otherwise contacts the underside of the piston crown as it reciprocates in the cylinder. This cooling which is generally accompanied by oil-jet spraying of the underside of the crown has seemed to provide a possible solution to the cooling problem. It should be realized that the provision of the oil-retaining structure within the piston crown can be a difficult and expensive matter, and as a result the use of two-piece pistons of most desireable configuration has been seriously curtailed.